Time administrators can use the Upload Times from Microsoft Excel function to perform a mass upload of employee times into the SAP Business ByDesign system for times that have been recorded outside the system and entered in a spreadsheet.

You can refer to Help center document - Upload Times Using Microsoft Excel

I have checked in our internal systems & yes, you can do modification to the previously released time sheets from Home Work center -> Self-Services Overview -> Edit Time Sheet -> From here you can select the released time sheets for a specific user & do changes to time sheet recordings for previous periods. (Please make sure that financial periods are open so that employee time calendar gets posted properly to account)